Seismic Swarm S20151223.1: Analysis of Activity Near Reno, Nevada

Seismic swarm S20151223.1 began at 06:18 on 23 December 2015 and concluded at 13:54 on 24 December 2015, centered 11 km south-southeast of Reno, Nevada. Over 31 hours and 36 minutes, the sequence produced 136 earthquakes. This event fits the established pattern of swarm activity in the region, where clusters of events occur without a single dominant mainshock.

Analysis of the first 100 events reveals a rapid onset with events concentrated in the initial hours. Magnitudes ranged from 0.1 to 4.4, with the largest shock recorded at 4.4 on 23 December at 06:46:07. Depths clustered tightly between 7 and 10 km, indicating a shallow crustal source typical of the local fault network. Early activity included several events above magnitude 3.0 within the first two hours, followed by a sustained sequence of smaller shocks. The temporal distribution showed peak rates immediately after initiation, with magnitudes generally decreasing over time while remaining above the detection threshold.

The Reno area occupies the western margin of the Basin and Range Province, where northwest-directed extension drives normal faulting along the Walker Lane belt. This tectonic setting accommodates shear between the Pacific and North American plates, producing frequent small-to-moderate earthquakes. Historical records document persistent swarm behavior linked to fluid migration and stress transfer along segmented faults. Since 2000, 29 swarms have been identified in the vicinity, occurring in 2003 (1), 2004 (1), 2005 (1), 2008 (7), 2010 (1), 2012 (4), 2013 (7), 2014 (4), and 2015 (3). These episodes underscore the region's elevated background seismicity without corresponding large-magnitude releases.

The 2015 swarm aligns with prior sequences in duration and depth range, reinforcing the role of the local fault system in accommodating ongoing extension. No surface rupture was associated with the activity, consistent with the modest energy release observed.

References SeismoSight internal swarm classification records USGS Earthquake Hazards Program regional tectonic summaries Nevada Seismological Laboratory fault database